This invention relates to tubeless pneumatic tires. More specifically, it relates to an innerliner (hereinafter innerliner, or liner, or liner ply) which is the inner-most layer of the tire structure for maintaining the internal air pressure in such a tire.
Prior to the present invention, in a typical pneumatic tire manufacturing operation several plies of reinforced rubber were laid up during the tire building step and these became permanently laminated during the molding and curing step. The inner-most one of these layers, the "innerliner", was composed of a rubber having lower air permeability than the rubber employed in other layers of the tire. This layer was usually based on butyl rubber or a halobutyl rubber, typically chlorobutyl.
In order to give a satisfactory performance in air pressure retention, the thickness of a halobutyl rubber based innerliner in a good quality auto tire must be on the order of about 60 mils. Such a liner has an air permeance of about 230 cc/m.sup.2 -atm-day at 65.degree. C. For a 15 in. size passenger tire or light truck tire the liner weighs about 2.5 lbs., which is about 10% of the total weight of the tire.
When a vehicle is running, all of the components in the tires are flexing at a high frequency. A certain amount of energy is consumed as a "loss" due to hysteresis, which is the major contributor to the rolling resistance of the tire and is indicated by the rise in the temperature of the material. A need exists in the tire industry for a reduction in the mass of any one or more components of the tire, without impairing the performance of the intended function of the component(s), that would reduce its share of the energy loss and result in an improvement of tire performance and fuel economy.
The rolling resistance of a tire can also be improved by increasing the air pressure in the tire. However, with the halobutyl rubber innerliner, the higher pressure can only be maintained by a thicker and heavier liner. The added weight of a thicker innerliner would offset the gain from the higher air pressure because it would increase the hysteresis loss. Therefore, the only meaningful way to achieve a higher tire pressure would be by improving the air barrier property of the innerliner. The present invention solves the above mentioned problems of the tire industry.
While butyl and halobutyl rubbers represent the best air barrier among elastomers, there are materials not normally considered to be elastomers which possess much better barrier properties. For example, vinylidene chloride based polymers (PVDC), and ethylene--vinyl alcohol copolymers (EVOH), have air permeabilities only a few percent or a fraction of a percent of that of the butyl rubbers. Although these barrier materials are well known and have been used in applications such as food packaging for their low air permeability, they have not been successfully used in tire construction.